High-purity quartz powder, process for producing the same, and glass molding

ABSTRACT

Subjects for the invention are to obtain a quartz powder having a high purity and high quality and a process for producing the same and to obtain a glass molding formed by melting and molding the powder and extremely reduced in bubble inclusion. 
     The invention provides a quartz powder, preferably a synthetic quartz powder obtained by the sol-gel method, which, upon heating from room temperature to 1,700° C., generates gases in which the amount of CO is 300 nl/g or smaller and the amount of CO 2  is 30 nl/g or smaller.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/75 8,395, filed on Jan. 16, 2004, now U.S. Pat. Ser. No. 7,063,826,which was a continuation of International Patent Application No.PCT/JP02/07322, filed on Jul. 18, 2002, and claims priority to JapanesePatent Application No. P2001-218997, filed on Jul. 19, 2001.

TECHNICAL FIELD

The present invention relates to a quartz powder having a high purityand high quality, a process for producing the same, and a glass moldingformed by melting and molding the powder and extremely reduced in bubbleinclusion.

BACKGROUND

In recent years, glass products for use in the field of opticalcommunication, semiconductor industry, and the like are required to haveexceedingly high quality. The purities thereof are strictly regulated.Such high-purity glasses are mainly produced by the known method (1) inwhich a sandy natural quartz powder obtained by pulverizing naturalquartz (this powder is generally called “sand”) is used as a rawmaterial. Methods usable in the case where a glass having a higherpurity is desired include: (2) the oxyhydrogen flame method whichcomprises decomposing silicon tetrachloride in an oxyhydrogen flame,depositing the resultant fume on a substrate to grow a fume deposit, andusing this fume deposit as a raw material; and (3) a method in which asynthetic quartz powder obtained by the so-called sol-gel method from agel formed from an organometallic compound such as, e.g., a metalalkoxide, is used as a raw material.

However, those methods each have had both merits and demerits. Forexample, in the method (1), since a natural quartz powder, whichintrinsically contains metallic elements such as aluminum and iron inthe quartz particles, is used as a raw material, it is difficult toobtain a quartz powder product highly purified to such a degree as tohave a metal impurity content of 100 ppb or lower even when apurification operation, e.g., pickling, is repeated. In the method (2),production at a commercially acceptable low cost is difficult, althougha high purity can be attained. This method hence has not come to be usedfor mass-production.

In the sol-gel method (3), on the other hand, there has been a drawbackthat the raw materials, intermediate, and product necessarily contactwith the production apparatus and impurities come into these due tocontacts with the apparatus, although the product can be mass-produced.In particular, the particles (sol or gel) and wet gel yielded byreactions of the organometallic compound as a raw material with analkoxide and water come into contact with the inner wall of theapparatus and repeatedly undergo deposition, peeling, and shedding,during which abnormal particles (scaling debris) come into the product.Examples of the apparatus in which such scaling debris generate includea series of synthetic-quartz production apparatus and individual devicesfor these which each have a part coming into contact with at least thereaction liquid, wet gel, or dry gel. Specific examples thereof includethe reactor, pulverizer, dryer, piping, and the like. It has beenextremely difficult to separate and remove the scaling debris from theproduct.

When the gel is burned to produce a synthetic quartz powder, suchscaling debris change into carbon ingredients. The carbon ingredientsaggregate to form black contaminant particles in the product. It hasfurther been known that when the synthetic quartz powder is melted informing a glass molding, the carbon ingredients decompose into gases andthese gases form bubbles in the glass molding and thereby significantlyimpair the quality of the glass molding.

An example of known methods for eliminating the problem described aboveis to regulate a synthetic quartz powder obtained by a sol-gel reactionso that the number of black particles present in the powder is reducedto 5 or smaller per 50 g (JP-A-8-188411). In this patent document, therealso is a statement to the effect that the glass molding obtained from amelt of this synthetic quartz powder is more reduced in bubble inclusionthan those obtained by related-art techniques.

However, the quality requirements which the glass products for use inthe field of optical communication, semiconductor industry, and the likeare required to satisfy are becoming severer. There has hence been adesire for the development of a synthetic quartz powder which iseffective in inhibiting bubble inclusion to a level lower than thatattained with such a related-art technique.

Another method is known for heightening the purity of a synthetic quartzpowder produced by the sol-gel method. This method comprisessufficiently supplying air in a burning step in order to reduce theamount of the scaling debris which are thought to have come into thepowder due to contacts with the production apparatus and which may comeinto the product and to prevent the scaling debris which have come intothe powder from remaining as unburned carbon. It has been known thatthis technique is effective in obtaining a glass molding reduced inbubble inclusion. However, even this technique cannot satisfy recentrequirements for a higher purity, and there has been a desire for asynthetic quartz powder effective in inhibiting bubble inclusion in ahigh degree. A method in which heat treatment is conducted under vacuumhas also been proposed. However, this method has had a problem thatindustrial use thereof is difficult, for example, because toindustrially realize vacuum conditions necessitates a high cost.

DISCLOSURE OF THE INVENTION

The present inventors made intensive investigations in order toeliminate the problems described above. As a result, they have foundthat when a quartz powder, in particular a synthetic quartz powderproduced by a sol-gel reaction, is one which, upon heating from roomtemperature to 1,700° C., generates gases in which the amount of CO is300 nl/g or smaller and the amount of CO₂ is 30 nl/g or smaller, thenthis quartz powder is the desired quartz powder having a high purity.They have further found that a glass molding extremely reduced in bubbleinclusion can be obtained from a melt of this quartz powder.

Furthermore, it has been found that this quartz powder is obtained as asynthetic quartz powder by hydrolyzing an alkoxysilane to obtain asilica gel having an average particle diameter of from 10 to 500 μm andbringing the silica gel into contact with helium and/or hydrogen gas ata temperature of from 400° C. to 1,300° C. The invention has been thuscompleted.

An essential point of the invention resides in a quartz powder which,upon heating from room temperature to 1,700° C., generates gases inwhich the amount of CO is 300 nl/g or smaller and the amount of CO₂ is30 nl/g or smaller.

Another essential point of the invention resides in a process forproducing a synthetic quartz powder which comprises hydrolyzing analkoxysilane to obtain a silica gel having an average particle diameterof from 10 to 500 μm and bringing the silica gel into contact withhelium and/or hydrogen gas at a temperature of from 400° C. to 1,300° C.

A still other essential point of the invention resides in a glassmolding obtained by melting and molding a quartz powder which, uponheating from room temperature to 1,700° C., generates gases in which theamount of CO is 300 nl/g or smaller and the amount of CO₂ is 30 nl/g orsmaller.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic view of an apparatus for producing vacuum glassampuls.

In the FIGURE, numeral 1 denotes a connection valve, 2 a full evacuationvalve, 3 a back pressure valve, 4 a leak valve, 5 a rough evacuationvalve, 6 a leak valve, 7 a diffusion pump, 8 a rotary pump, 9 anionization vacuum gauge, and 10 a Geissler's tube, and symbols a, b, c,and d are glass ampul connection ports.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be explained below in detail.

The quartz powder of the invention is a high-purity quartz powder which,upon heating from room temperature to 1,700° C., generates gases, in allof which CO amounts to 300 nl/g or smaller and CO₂ amounts to 30 nl/g orsmaller. In particular, the quartz powder preferably is one which, uponheating from room temperature to 1,700° C., generates gases, in all ofwhich N₂ amounts to 50 nl/g or smaller and H₂ amounts to 150 nl/g orsmaller.

For determining gases generated by heating the quartz powder, anydesired technique among generally known ones can be used. Preferred ofthese is a technique with which even a gas generated in an exceedinglysmall amount can be detected and analysis is possible underhigh-temperature conditions. Examples thereof include TPD-MS(temperature programmed desorption-mass spectroscopy).

In the invention, the gases generated are determined in the followingmanner. The quartz powder to be analyzed (sample) is placed in a sealedvacuum glass ampul. This ample is heated from room temperature to 1,700°C., and the gases generated during this heating are determined.

The specific procedure is as follows. A quartz powder sample is placedin an ampul made of glass, and the pressure inside the ampul istemporarily regulated to 1.3×10⁻³ to 1.3×10⁻⁴ Pa. Subsequently, thesample is heated at 200° C. for 10 minutes and then allowed to stand for1 hour in order to removed adsorbates including water at an increasedrate. After the degree of vacuum in the ampul is ascertained to be keptstable at 1.3×10⁻³ to 1.3×10⁻⁴ Pa, the tip is melted to seal the ampul.Thus, a vacuum ampul is produced.

The quartz powder of the invention is characterized in that when thequartz powder is heated from room temperature to 1,700° C., it generatesgases, in all of which CO amounts to 300 nl/g or smaller and CO₂ amountsto 30 nl/g or smaller. These temperature conditions are preferredbecause gases can be generated in the maximum amounts while preventingthe particles of the quartz powder from sintering.

In the invention, the shorter the time period required for heating thequartz powder from room temperature to the final temperature (heatingrate), the better. Specifically, it is preferred that the quartz powderbe heated to the final temperature in 10 minutes, preferably in 5minutes. Such rapid heating is preferred because the determination ofthe gases generated from the synthetic quartz powder during the rapidheating gives stable results. Too long heating rates may result influctuations in determination results.

It is preferred that the quartz powder of the invention have a bulkdensity of from 1.3 to 1.7 g/cm³ and a metal impurity content of 500 ppbor lower, because this quartz powder gives a glass molding having afurther reduced bubble content.

Although the bulk density preferably is 1.3 g/cm³ or higher, too lowvalues thereof may result in a large volume contraction and reduceddimensional stability in melting/molding the quartz powder. On the otherhand, there are cases where quartz powders having a bulk densityexceeding 1.7 g/cm ³ are difficult to produce.

The term metal impurity content as used for the quartz powder of theinvention means the content of all metal impurities, which includealkali metals, alkaline earth metals, aluminum, iron, and copper. Fordetermining the content thereof, any desired technique may be used. Thecontent of metal impurities in the quartz powder of the invention ispreferably 200 ppb or lower, especially 100 ppb or lower.

For producing the quartz powder of the invention, e.g., the syntheticquartz powder, any desired method may be used. However, it is preferredto produce the powder by the sol-gel method. In particular, it ispreferred to produce the powder by a process which comprises hydrolyzingan alkoxysilane to obtain a silica gel having an average particlediameter of from 10 to 500 μm and bringing the silica gel into contactwith helium and/or hydrogen gas at a temperature of from 400° C. to1,300° C.

The average particle diameter of the silica gel to be brought intocontact with helium and/or hydrogen gas is preferably from 10 to 500 μm,especially from 100 to 500 μm. When the average particle diameterthereof is too small, gas adsorption or the like is apt to occur becauseof the increased surface area of the particles. The gas adsorption iscausative of bubbles in glass moldings. Conversely, too large averageparticle diameters may also be causative of bubbles in glass moldingsbecause the efficiency of impurity removal from such particles is low.

The temperature at which the silica gel is contacted with helium and/orhydrogen gas is preferably from 600° C. to 1,300° C., especiallypreferably from 800° C. to 1,300° C.

The helium and/or hydrogen gas with which the silica gel is contactedpreferably is a helium/hydrogen mixed gas containing up to 4% hydrogen.Especially preferably, the silica gel is contacted with pure helium gas.Any desired method may be used for contacting with helium and/orhydrogen gas. Examples thereof include a method in which the gas ispassed through the silica gel and a method in which the silica gel isenclosed in a container together with helium and/or hydrogen gas andheld therein in an atmosphere of helium and/or hydrogen gas at ordinaryor an elevated pressure.

In the production of a synthetic quartz powder by the sol-gel method,the contacting of helium and/or hydrogen gas with the silica gel havingan average diameter of from 10 to 500 μm can be conducted at any desiredtime. For example, use may be made of a method in which the silica gelwhich has been cooled to room temperature is reheated and contacted withhelium and/or hydrogen gas at a temperature of from 400° C. to 1,300° C.or a method in which a silica gel burning step is partly or whollyconducted in a helium and/or hydrogen gas atmosphere. Furthermore, whena synthetic quartz powder obtained as a product through burning issubjected to a heat treatment again, this powder may be contacted withhelium and/or hydrogen gas.

Preferred is a method in which the silica gel is heat-treated at 1,000°C. or higher for from 10 to 50 hours in an oxygen-containing atmospherebefore or after it is contacted with helium and/or hydrogen gas. Thetemperature for this heating is preferably not lower than thevitrification temperature, especially preferably not lower than 1,200°C. The time period of this heat treatment conducted under suchtemperature conditions is preferably from 20 to 40 hours, especiallypreferably from 25 to 35 hours.

Examples of methods for the heat treatment in an oxygen-containingatmosphere include a method in which the heat treatment is conductedwhile passing dry air. The time period which should be taken to heat tothe heat treatment temperature (heating rate) is not limited. However,the heating rate is generally from 50 to 200° C./hr, preferably from 70to 150° C./hr.

The above-described heat treatment of the silica gel in anoxygen-containing atmosphere and the contact treatment thereof withhelium and/or hydrogen gas at from 400° C. to 1,300° C. may be conductedin any order. It is, however, preferred to conduct the heat treatment inan oxygen-containing atmosphere before the contact treatment with heliumand/or hydrogen gas is performed.

The two treatments (the heat treatment in an oxygen-containingatmosphere and the contact treatment with helium and/or hydrogen gas)may be alternately conducted in portions each. For example, a silica gelobtained by the sol-gel method is heated to around 800° C. in a dry airstream and then heated to 1,700° C. in a stream of helium and/orhydrogen gas in place of the dry air. At the time when from 10 to 50% ofthe first half of the whole period of the heat treatment at the finaltemperature has passed, the stream is changed to dry air again. The heattreatment is then wholly carried out at the final temperature. At thetime when the heating has been completed or immediately before thecompletion thereof, the stream is changed to helium and/or hydrogen gasagain. Thereafter, the stream may be changed to dry air once more.

When the quartz powder of the invention, e.g., the synthetic quartzpowder obtained in the manner described above is heated from roomtemperature to 1,700° C., it generates gases including CO, CO₂, H₂, andN₂ in reduced amounts. Although the reasons for this are unclear, thefollowing is thought. Molecules of a small size, such as helium andhydrogen, sufficiently dissolve in the quartz particles and have a highdiffusion velocity. These molecules hence expel gases which have a largemolecular size and are especially less apt to diffuse in the particles,e.g., CO, CO₂, and N₂, from the particles to thereby bring about thereduced gas generation. It is further thought that as a result of thediminution of CO and CO₂, the water in the synthetic quartz powder (thewater which is generated by an equilibrium reaction between a silanolgroup (≡SiOH) and silica (SiO₂) in the quartz powder) and the H₂ whichis generated by an equilibrium reaction among these (CO+H₂O⇄CO₂+H₂) aresimultaneously diminished.

In the case where the sol-gel method is used in producing the syntheticquartz powder of the invention, the production conditions may besuitably selected as long as a silica gel having an average particlediameter of from 10 to 500 μm obtained by the hydrolysis of analkoxysilane is brought into contact with helium and/or hydrogen gas at400° C. to 1,300° C. as described above.

A specific procedure is, for example, as follows. An alkoxysilane andhighly pure water are introduced into a reactor. The amount of the purewater to be introduced is from 1 to 10 equivalents to the alkoxysilane.A sol-gel reaction is conducted. Thereafter, the reaction product (wetgel) is pulverized to an average particle diameter of from 10 to 500 μmand dried to obtain a silica gel (dry gel) as a silica precursor. As thealkoxysilane can be used any desired alkoxysilane capable of giving analkoxysilane oligomer through a hydrolytic polycondensation reaction,such as tetramethoxysilane, tetraethoxysilane, or the like.Tetraalkoxysilanes are preferred and tetramethoxysilane is especiallypreferred.

An organic solvent compatible with water, such as an alcohol or ether,may be mixed as a solvent in the hydrolytic condensation reaction.Furthermore, a catalyst such as an acid or alkali may be used in orderto accelerate this reaction. Preferred are catalysts containing nometal. In general, an organic acid, ammonia water, or the like ispreferred.

The reactor may be heated or cooled in order to control the gelation ofthe hydrolyzate. The wet gel obtained by this reaction is pulverized soas to have a regulated particle size. The particle size distributionobtained by this pulverization governs the particle size distribution ofthe synthetic quartz powder as the final product. It is important thatthe optimal particle size of the wet gel be determined from the targetparticle size distribution of the product while taking account of theparticle contraction to be caused by drying and burning. The silica gelafter drying (dry gel) preferably has an average particle diameter ofgenerally from 10 to 500 μm, preferably from 90 to 500 μm, especiallyfrom 100 to 500 μm. The degree of drying of the gel is generally from 1to 30% by weight in terms of water content. Such drying is generallyaccomplished by heating to 100 to 200° C. at a reduced pressure or in aninert gas atmosphere.

The dry gel thus produced is brought into contact with helium and/orhydrogen gas at a temperature of from 400° C. to 1,300° C. to obtain ahigh-purity synthetic quartz powder. In general, however, the resultantpowder is further burned usually for from 10 to 100 hours at variedtemperatures in the range of from 400° C. to 1,250° C. to make thepowder particles non-porous. Thus, a high-purity synthetic quartz powderis obtained.

In the invention, a fusion test was conducted in order to ascertainbubble inclusion in a glass molding. Quartz powders are melted by themethod of fusion with an oxyhydrogen flame, i.e., Verneuil's method. Thebubbles present in each ingot produced are counted. Based on the counts,the powders can be compared in the tendency to generate bubbles.

EXAMPLES

The invention will be explained below in more detail by reference toExamples. However, the invention should not be construed as beinglimited to the following Examples unless the invention departs from thespirit thereof.

Examples 1-1 and 1-2

Into a mixing vessel were introduced tetrame thoxysilane and water. Theamount of the water was 5 equivalents to the silane. The contents werestirred at a temperature of 30° C. for 1 hour to obtain a homogeneoussol solution through a hydrolysis reaction. This solution wastransferred to a vat made of poly(vinyl chloride) and allowed to standfor 5 hours to cause it to gel. This gel was dried for 12 hours with a140° C. vacuum dryer and then subjected to particle size regulation soas to result in an average particle diameter of 320 μm.

One kilogram of the dry gel powder thus obtained was introduced into alidded container made of quartz glass. This container was set in anelectric furnace, and a gas introduction nozzle was inserted into a holeformed in the lid. While dry air was kept being passed, the powder washeated to 1,200° C. at a heating rate of 100° C./hr and held at 1,200°C. for 30 hours to thereby burn the powder. The passing of dry air wascontinued until the powder cooled sufficiently. Thus, a synthetic quartzpowder was obtained.

A 500-g portion of the high-purity synthetic quartz powder obtained wasintroduced into a lidded container made of quartz glass. This containerwas set in an electric furnace, and a gas introduction nozzle wasinserted into a hole formed in the lid. While helium gas was kept beingpassed, the powder was heated to 1,200° C. at a heating rate of 400°C./hr and held at 1,200° C. for 10 hours to thereby burn the powder. Thepassing of helium gas was continued until the powder cooledsufficiently.

A 100-g portion of the helium gas-treated synthetic quartz powder thusobtained was placed in a 100-ml measuring cylinder made of glass tomeasure the bulk density (tap density) thereof. As a result, the densitywas found to be 1.34 g/cm³. This synthetic quartz powder was furtherexamined for total metal impurity content. The method of measurement wasas follows. First, the synthetic quartz powder was dissolved inhigh-purity hydrofluoric acid and this solution was heated andevaporated to dryness. Subsequently, this dry solid was dissolved in a10:1 (by volume) liquid mixture of high-purity dilute nitric acid andhigh-purity dilute sulfuric acid. The solution thus obtained wasexamined for metal impurity content by ICP-MASS. As a result, the metalimpurity content was found to be about 57 ppb. The content of each metalis shown in Table 2.

TABLE 2 Kind of metal Content (ppb) Na 10 K 2 Ca 5 Al 1 Fe 20 Cu <0.5 Mn<0.5 Ti <1 P 5 As 0 B 3 Ge 9 U <0.06 Th <0.3

Subsequently, part of this helium gas-treated synthetic quartz powderwas placed in sealed vacuum ampuls by the following method. These ampulswere heated to 1,700° C. at a heating rate of 20° C./min to measure theamounts of gases generated during the heating.

Ampul Production

Test tube type enclosures (eight test tube type enclosures: made ofquartz; inner diameter, about 8 mm, thickness, about 1 mm; length, about130 mm) were washed with a neutral detergent and distilled water.Thereafter, the enclosure tubes were subjected to acetone displacementand dried with nitrogen introduction. A1.02-g portion was weighed out ofthe synthetic quartz powder sample obtained by the method describedabove, and placed in each enclosure tube. The enclosure tube wasthinned, with heating with a burner, in a part thereof located about 5cm apart from the bottom. Thereafter, these enclosure tubes wereconnected by welding to the glass ports a to d of a vacuum apparatus(MODEL EH-2A, manufactured by TOKUDA) and evacuated. A diagrammatic viewof the vacuum melting apparatus is shown in FIG. 1 (ampuls are notshown). As an ionization vacuum gauge was used MODEL HFT-4, manufacturedby TOKUDA. Of the four ampuls, the ampuls connected to the glass ports aand b shown in FIG. 1 contained the synthetic quartz powder and theampuls connected to the glass ports c and d contained nothing so as toserve as blanks.

The rotary pump 8 was operated to reduce the pressure in the system.After the Geissler's tube 10 was ascertained to hardly luminesce, theback pressure valve 3 was opened. At about 15 minutes after the openingof the back pressure valve 3, the diffusion pump 7 began to be operatedand an operation for elevating the degree of vacuum was initiated so asto further elevate the degree of vacuum in the system. At 10 minutesafter initiation of the operation of the further elevation of the degreeof vacuum, the degree of vacuum as measured with the ionization vacuumgauge 9 (TOKUDA MODEL HFT-4) was 5.3×10⁻³ Pa.

Subsequently, the ampuls were heated at 200° C. for 10 minutes in orderto remove adsorbates including water at an increased rate. At 60 minutesafter the initiation, the degree of vacuum was stable at 2.7×10⁻³ Pa.The connection valve 1 was closed and the degree of vacuum in the vacuumapparatus system only was ascertained. As a result, no change wasobserved. The connection valve 1 was opened, and the opening of eachampul was heated with a burner and sealed. These ampuls were separatedfrom the glass ports to produce vacuum glass ampuls. During thisoperation, the degree of vacuum in the system was stable at 2.7×10⁻³ Paand no change was observed.

Determination of Amounts/Kinds of Gases Generated

A small vacuum/pressurizing furnace (Type: FVPHP-R-5, FRET-35)manufactured by Fuji Denpa Kogyo K. K. and a crucible made of carbonwere used. In this crucible were placed the quartz glass ampuls producedby the method described above. These ampuls were heated to 1,700° C. ata rate of 20° C./min and a degree of vacuum of 40 Pa and kept beingheated for 20 minutes. After the glass ampuls were cooled and thepressure was returned to ordinary pressure, the ampuls were taken out.

Determination of Gases in Ampuls

The ampuls were set in a fracture vessel which had openings connected toan evacuation pump and a mass spectrometer and the system in which wassealable. The vessel was evacuated with the evacuation pump. Thereafter,the valve disposed at the gate to the mass spectrometer (AGS7000,manufactured by Anerupa K. K.) attached to the fracture vessel wasopened. The degree of vacuum in the vessel was further elevated (to1.3×10⁻³ Pa) and this vacuum was maintained for 1 hour or more.

Subsequently, the mass spectrometer was operated at a detector voltageof 2,600 V, and the ampuls were fractured while monitoring m/z of 2(hydrogen), 4 (helium), 18 (water), 28 (nitrogen and CO), 30 (NO), 32(oxygen), and 44 (carbon dioxide). The nitrogen and CO weredistinguished from each other based on the proportion of m/z=12 tom/z=14.

Gases are discharged from the ampuls to give peaks. The gases generatedwere determined from the areas of the resultant peaks based on therelative sensitivity ratios to the peaks for standard substances orstandard gases.

This determination of gases generated was conducted twice respectivelyin Examples 1-1 and 1-2. The results are shown in Table 1.

TABLE 1 Volume of ampul after heating Kind/amount of gas generated(nl/g) (cc) H₂ H₂O N₂ CO O₂ CO₂ Total Comparative 7.2 177 1 79 388 0 26670 Example 1 Example 1-1 4.2 100 — 10 160 0 1 271 Example 1-2 4.2 53 —10 133 0 2 198 Comparative 4.1 77 1 16 177 0 4 274 Example 2

As apparent from Table 1, the ampuls showed only a slight volumeexpansion through the heating and gases were generated in reducedamounts. The differences in found values between the two measurementswere small, showing that the precision of the analysis was excellent.

Furthermore, the helium gas-treated synthetic quartz powder obtained bythe method described above was used to produce an ingot having adiameter of 12 mm and a height of 60 mm using a Verneuil's-methodmelting apparatus employing an oxyhydrogen flame. In a dark room, thisingot was illuminated by a flashlight, and the bubbles were visuallycounted through a magnifying lens. As a result, the number of bubbleswas as small as 3.

Comparative Example 1

A synthetic quartz powder was obtained in the same manner as in Example1, except that the treatment with helium gas was omitted. The amounts ofthe CO, CO₂, and other gases generated were determined in the samemanner as in Example 1. The results are shown in Table 1.

As apparent from Table 1, the ampuls underwent a large volume expansionthrough the heating and gases were generated in large amounts.

Furthermore, this synthetic quartz powder was used to produce an ingotin the same manner as in Example 1. This ingot was illuminated by aflashlight in a dark room and the bubbles were counted. As a result, thenumber of bubbles was as large as 42. It was hence apparent that thequartz powder poses a problem when formed into a glass molding, e.g., acrucible.

Comparative Example 2

A synthetic quartz powder was obtained in the same manner as in Example1, except that a heat treatment at 1,150° C. was conducted at a degreeof vacuum of from 30 to 100 Pa for 8 hours in place of the treatmentwith helium gas. The amounts of the CO, CO₂, and other gases generatedwere determined in the same manner as in Example 1. The results areshown in Table 1.

Table 1 shows the following. The amounts of the gases generated could bereduced as in the case of the helium gas-treated synthetic quartzpowder. However, this method was unsuitable for industrial use becausenot only the treatment required much time but also it was necessary tokeep the inside of the heat treatment apparatus under vacuum conditions.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on a Japanese patent application filed on Jul.19, 2001 (Application No. 2001-218997), the entire contents thereofbeing hereby incorporated by reference.

INDUSTRIAL APPLICABILITY

According to the invention, a high-quality synthetic quartz powderreduced in bubble generation upon melting and a glass molding can beobtained.

The glass molding obtained from a melt of the synthetic quartz powder ofthe invention has high quality with considerably reduced bubbleinclusion. The reasons for this are thought to be probably as follows.

To inhibit the formation of black particles has been possible in somedegree with a related-art technique which comprises sufficiently passingair or regulating oxygen concentration to thereby accelerate combustionin order to remove the carbon ingredients from scaling debris (abnormalparticles) which have come into. However, there is a possibility thatcarbon compound aggregates which are clusters formed by the partialgathering of carbon ingredients might remain in the quartz powder eventhough they are not completely carbonized into black particles. It isknown that the carbon present in such abnormal particles, which have anincreased carbon ingredient concentration, displaces silicon atoms ofthe silica framework in the synthetic quartz and is thereby incorporatedinto the framework. The carbon which has been incorporated into theframework is expected to be highly difficult to remove. Vitrification ofquartz powders generally requires a temperature as high as 1,700° C. orabove. In case where a synthetic quartz powder produced at a lowertemperature than that contains residual carbon ingredients which havebeen incorporated therein, these carbon ingredients are thought to yieldCO, CO₂, etc. during high-temperature melting to cause bubble inclusionin the glass molding. The gases, such as CO and CO₂, released from thequartz powder particles are thought to highly relate to the carboningredients contained in the scaling debris in the product. A syntheticquartz powder which generates CO and CO₂ in small amounts upon heatingto a high temperature is thought to be reduced in the content of carboningredients in the scaling debris in the powder and be effective inreducing bubble inclusion in obtaining a glass molding from a meltthereof.

1. A quartz powder which, upon heating from room temperature to 1,700°C., generates gases in which the amount of CO is 300 nl/g or smaller andthe amount of CO₂ is 30 nl/g or smaller, wherein said quartz powder hasa bulk density of from 1.3 to 1.7 g/cm³ and a metal impurity content of500 ppb or lower, wherein said quartz powder is a synthetic quartzpowder produced by a sol-gel method, which comprises: (a) forming asilica gel by a sol-gel method; and (b) subjecting said silica gel to aheat treatment in an oxygen-containing atmosphere and subjecting saidsilica gel to contact with helium gas, wherein said subjecting saidsilica gel to a heat treatment in an oxygen-containing gas is carriedout either before or after subjecting said silica gel to contact withhelium gas.
 2. The quartz powder of claim 1, wherein said silica gel iscontacted with helium gas at a temperature of from 400° C. to 1,300° C.3. The quartz powder of claim 1, wherein said silica gel is subjected toa heat-treated at a temperature of 1,000° C. or higher for from 10 to 50hours in an oxygen-containing atmosphere.
 4. The quartz powder of claim1, wherein said synthetic quartz powder has a metal impurity content of200 ppb or lower.
 5. The quartz powder of claim 1, wherein saidsynthetic quartz powder has a metal impurity content of 100 ppb orlower.
 6. The quartz powder of claim 1, wherein said silica gel isbrought into contact with helium gas at a temperature of from 600° C. to1,300° C.
 7. The quartz powder of claim 1, wherein said silica gel isbrought into contact with at least one of helium and hydrogen gas at atemperature of from 800° C. to 1,300° C.
 8. The quartz powder of claim1, wherein said silica gel has an average particle diameter of from 100to 500 μm.
 9. The quartz powder of claim 1, wherein said silica gel isbrought into contact with a mixture of helium and hydrogen gas.
 10. Thequartz powder of claim 9, wherein said mixture of helium and hydrogengas comprises up to 4% hydrogen.
 11. The quartz powder of claim 1,wherein said silica gel is heat-treated at a temperature of 1,200° C. orhigher for from 10 to 50 hours in an oxygen-containing atmosphere. 12.The quartz powder of claim 1, wherein said silica gel is heat-treated ata temperature of 1,000° C. or higher for from 20 to 40 hours in anoxygen-containing atmosphere.
 13. The quartz powder of claim 1, whereinsaid silica gel is heat-treated at a temperature of 1,000° C. or higherfor from 25 to 35 hours in an oxygen-containing atmosphere.
 14. Thequartz powder of claim 1, wherein said silica gel is heat-treated at atemperature of 1,000° C. or higher for from 10 to 50 hours in anoxygen-containing atmosphere at a heating rate of 50 to 200° C. perhour.
 15. The quartz powder of claim 1, wherein said silica gel isheat-treated at a temperature of 1,000° C. or higher for from 10 to 50hours in an oxygen-containing atmosphere at a heating rate of 70 to 150°C. per hour.
 16. A glass molding, which is produced by melting andmolding a quartz powder of claim 1.